Morphological heterogeneity and phenotypical instability versus metastatic stability in the murine tumor model ER 15-P

Summary At clinical presentation, the majority of malignant tumors are composed of multiple clonal subpopulations of tumor cells with different phenotypic characteristics. Using the experimental tumor model ER 15-P, a methylcholanthrene-induced pleomorphic sarcoma of the C57 Bl₆J mouse, we studied a system of long-term in vivo passages of this primary tumor for cell morphological changes, and alterations in the potential for spontaneous lung metastases. Transplants from the primary after the 4th, 20th, 40th and 80th i.m. passage (referred to as T4, T20, T40, and T80 respectively) together with their lung metastases were investigated by light microscopy, immunohistochemistry, and electron microscopy. In addition, the potential for metastasis to the lungs in each group was determined and compared with that of the parent T4 tumors. T4 tumors were mainly composed of spindle-shaped tumor cells with the ultrastructural features of fibroblasts and myofibroblasts, often arranged in a storiform or fasciculated growth pattern, and intermingled with tumor giant cells. Some small areas contained polygonal or rounded tumor cells, ultrastructurally undifferentiated, and sometimes arranged in a hemangiopericytoma-like growth pattern. Although electron-microscopical findings clearly demonstrated the mesenchymal origin of these tumor cells, immunostaining with a polyclonal antibody to vimentin was unspecific in all tumor cells and normal mouse tissue. Monoclonal antibodies to vimentin from different sources were completely negative in tumor cells and murine stromal components. In contrast, myofibroblast-like tumor cells showed immunohistochemically, a moderate to strong co-expression with monoclonal antibodies to desmin, muscle actin and -smooth muscle actin. On the basis of these morphological findings, the primary ER 15-P was classified as a pleomorphic myofibrosarcoma. The lung metastases of T4 tumors were mainly composed of undifferentiated round to polygonal tumor cells, while the number of desmin-positive, muscle- and -smooth muscle-actin-positive cells was reduced. The morphological features of T20 tumors and their lung metastases were the same as in T4, indicating a relative stability of the phenotype up to that stage. In contrast, T40 and T80 tumors and their lung metastases were found to contain almost exclusively undifferentiated tumor cells and many tumor giant cells. While fibroblast-like tumor cells were seen only occasionally, myofibroblast-like tumor cells had almost completely disappeared. The potential for lung metastases was nearly constant in all groups, suggesting metastatic stability. Obviously, the undifferentiated tumor cells of this model are associated with a higher metastatic potential.Abbreviations T4, T20, T40, T80 transplants from the primary tumor after the 4th, 20th etc. i.m. passage - MFH malignant fibrous histiocytoma